Fuel cells have recently received attention as clean energy. Hydrogen gas serving as a fuel for the fuel cells is not present in a large amount in nature and should be generated artificially.
As one of processes for the hydrogen generation, there is a process for preparing hydrogen by electrolysis of water. This technique, however, costs too much at current technical levels, and hydrogen production is therefore currently mainly performed by reforming of fossil resources.
This process, however, gives not only hydrogen but also impurity gases such as CO, CO2 and H2O. Among them, CO poisons electrodes of the fuel cells. To avoid this and to adopt hydrogen obtained by reforming of the fossil resources to fuel cells, hydrogen should be separated and purified from such impurity gases to have a higher purity.
A membrane separation technique using a metal membrane is known as a hydrogen purification technique for simply obtaining high-purity hydrogen. Exemplary hydrogen separation metal membranes now practically used include a Pd—Ag alloy membrane. However, it is predicted that the Pd—Ag alloy membrane does not satisfy growing demands made upon wider usage of fuel cells in future, because expensive and rare Pd poses limitations to the use of Pd—Ag alloy membrane. For this reason, there is a demand for developing a novel metal membrane material instead of the Pd—Ag alloy.
As novel metal membrane materials, V, Nb and Ta have high hydrogen permeability even being used alone and receive attention. Hydrogen separation alloys having both high hydrogen permeability and satisfactory hydrogen-embrittlement resistance have been developed by alloying any of these metals with another metal such as Ti, Zr, Hf, Ni or Co to form a multiphase alloy.
Typically, Ni—Ti—Nb multiphase alloys proposed in PTL 1 and PTL 2 each including a phase playing a role in hydrogen permeation and a phase playing a role in resistance to hydrogen embrittlement become a focus of attention.
The multiphase alloys described in PTL 1 and PTL 2 each have a composition including only two phases of a phase having satisfactory hydrogen permeability but becoming brittle and susceptible to fracture upon hydrogen absorption (hereinafter also referred to as a “hydrogen-permeable phase”) and a phase having inferior hydrogen permeability but being resistant to fracture even upon absorption of hydrogen (hereinafter also referred to as a “hydrogen-embrittlement-resistant phase”), or a composition to form an eutectic crystal of the two phases.
The present inventors have proposed a hydrogen permeable alloy having an oxygen content of 1000 ppm or less in a cast state as a technique for improving the workability of the aforementioned Ni—Ti—Nb alloy in PTL 3.
Independently, PTL 4 discloses a multiphase hydrogen permeable alloy in which a phase playing a role in hydrogen permeation extends in a hydrogen-permeation direction as a technique for improving the hydrogen permeability coefficient by regulating the metallographic structure of the alloy.